The invention relates to a method for laser drilling holes in a substrate, in particular an electrical circuit substrate, the laser beam with a spot diameter smaller than the hole diameter to be drilled being moved along at least one circular path in the region of the hole to be drilled in each case.
U.S. Pat. No. 5,593,606 discloses a method of this kind whereby holes with a diameter larger than the beam diameter of the laser are produced by moving the laser beam outward or inward either along spiral paths or in concentric circles within the hole region.
For the drilling of circuit boards or comparable circuit substrates, after optimization the particular deflector unit used is moved to one hole position after another, the shape and processing or machining of the hole being expediently predefined by a program (a so-called drilling tool). Using this program each hole is machined in an identical manner. In a suitable procedure, for particularly precise and rapid drilling the laser beam is programmed to jump from an initial position, e.g. a previously drilled hole, to the center of the new hole to be drilled and is traversed from there to the first circular path in an always identical, defined angular direction. After completing this first circular path, the beam can then, if necessary, be traversed to other circular paths. If the traversing direction from the hole center to the first circular path is now fixed at a predefined angle, whereas the previous jump direction from different starting points (drilled holes) assumes quite different angles in each case, this means that for the majority of all holes a change of direction of up to 180° is necessary between the jump to the hole center and the traversing movement to the circular path. The larger this change of direction between the jump direction and the traversing direction, the greater the necessary abrupt change in position of the deflector unit also, i.e. in general the galvo mirrors. An angular change of direction always means a time loss due to the necessary recovery time of the mirrors after the movement as well as a stress factor for the galvo motors because higher peak currents are flowing; this has a negative effect on their service life. If a mirror recovery time is dispensed with, this adversely affects hole quality, i.e. non-circular holes are obtained. In addition, particularly in the case of a 180° reversal in the direction of travel, it is first necessary to stop, and then re-accelerate, which means that a new galvo motor lag error is introduced.